I have come up with a somewhat schizo theory. I think part of the story regarding Fecto is fake and is actually revisionist history driven by cope, and he had a way bigger impact to the world and story than what is apparent.

From the figure description about the meteors it is now known that what we see in the game is the only remaining habitable land in the planet, and the rest somehow sunk.
What I think happened: Fecto actually managed to wreck most of the planet, probably both by arriving in a meteor with a massive crash and then personally attacked and razed the rest. The fake part of the story is that Fecto was defeated and contained and then Elfilin was created later. Actually Fecto was in course to destroy all life until Elfilin somehow grew powerful enough to separate on his own, which left him in a weakened state which is how his capture was possible. The fake story was created to cope with the fact most of life on the planet has been killed off with no way to recover since the land itself has also disappeared, and a way to control the narrative was needed. It's confirmed that Wondaria was created as a huge cope-fest propaganda machine so we know this civilization is willing to lie to itself. Also the reason why the planet was abandoned was not because they figured they would just find something better just by leaving, but because Fecto's attack destroyed the economy and food production, and they were going to go nearly extinct and regress to the stone ages if they didn't leave.